Cancer is a major and often fatal disease. Accordingly, efforts to develop new therapies for cancer is a constantly ongoing effort of the research society. The vast majorities of cancers are present as solid tumours, e.g. lung cancer, breast cancer, prostate cancer, while the rest are hematological and lymphoid malignancies, e.g. leukemias and lymphomas.
Chemotherapy is used in attempts to cure or palliate the disease. In most cases this therapy is delivered in the form of combination chemotherapy, when two or more drugs having different modes of action are used together in order to optimise the antitumoural effect and to minimise side effects. The results obtained with chemotherapy vary according to tumour type. Some tumours are very sensitive and the treatment has then a high probability of leading to cure. Examples of this type of tumours are acute leukemias, malignant lymphomas, testicular cancer, chorion carcinomas and Wilms tumour. In another group of tumours chemotherapy can result in good palliation and prolonged survival. Examples of such tumours are breast cancer, colo-rectal cancer, ovarian cancer, small-cell lung cancer, bladder cancer, multiple myeloma, and chronic leukemias of both lymphatic and myeloid type. Primary drug resistant tumours are for instance malignant glioma, melanoma, prostate cancer, sarcomas and gastrointestinal tumours other than colo-rectal cancers.
Alkylating agents, such as drugs derived from nitrogen mustard, that is bis(2-chloroethyl)amine derivatives, are used as chemotherapeutic drugs in the treatment of a wide variety of neoplastic diseases. These drugs all act by covalent interaction with nucleophilic heteroatoms in DNA or proteins. It is believed that these difunctional agents are able to crosslink a DNA chain within a double helix in an intrastrand or interstrand fashion, or to crosslink between DNA and proteins. The crosslinking results in inhibitory effects on DNA replication and transcription with subsequent cell death. The drugs can be used as single agents or in combination with other antineoplastic agents. Alkylating agents seem to have some propensity for fast growing tissues. They exert effects in a broad spectrum of tumours. Side effects are mainly restricted to bone marrow and at very high doses also to the gastrointestinal tract.
Melphalan, or p-bis-(2-chloroethyl)-aminophenylalanine, is a conjugate of nitrogen mustard and the amino acid phenylalanine, which was synthesised in the mid 1950s (U.S. Pat. No. 3,032,584). This classic alkylating substance soon became a valuable drug in the chemotherapeutic field and is still of importance in the treatment of for example myeloma. Melphalan was originally developed as a selective cytotoxic agent against melanoma cells, which utilise large amounts of phenylalanine in melanin synthesis. Clinical use of melphalan in the treatment of metastatic melanomas has, however, had limited efficacy.
In the search for a more selective action on malignant cells melphalan analogues have been synthesised. Sarcolysine, m-bis-(2-chloroethyl)aminophenylalanine, was obtained by shifting the bis-(2-chloroethyl)-amino group from the para- to the meta-position of phenylalanine. By covalent conjugation of different amino acids at the amino and carboxylate groups of sarcolysine a peptide mixture known as Peptichemio® (PTC) was prepared. PTC consisted of six different peptides (de Barbieri, “Proceedings of the symposium on Peptichemio”, Milan, Nov. 18, 1972). PTC was subsequently shown to be active on several tumour types as well as on tumours resistant to treatment with alkylating agents including melphalan and entered clinical trials with promising results. For the understanding of the effects and usage of PTC a serious disadvantage was the fact that it is a mixture of six peptides. The cytotoxic effects of each of the different peptides contained in PTC were therefore measured separately (Lewensohn et al., Anticancer Research 11: 321–324 (1991)) and a wide variation in the cytotoxicities of the peptides was found. One of the peptides, L-prolyl-m-L-sarcolysyl-L-p-fluorophenylalanine ethyl ester hydrochloride (P2) turned out to be more toxic to RPMI 8322 melanoma cells than any of the other peptides.